An anterior radio frequency (rf) coil array for a magnetic resonance imaging (mri) system

ABSTRACT

Various methods and systems are provided for a flexible, lightweight, low-cost radio frequency (RF) coil array of a magnetic resonance imaging (MRI) system. In one example, a RF coil array for a MRI system includes a plurality of RF coils, each RF coil comprising an integrated capacitor coil loop; a plurality of coupling electronics units each coupled to a respective coil loop; and a plurality of wires coupling each coupling electronics unit to an interface board configured to couple to a cable of the MRI system. The RF coil array is a high density (referring to the number of coil elements) anterior array (HDAA) or a high definition (referring to image resolution) anterior array.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication No. 62/426,028, filed on Nov. 23, 2016, the entirety ofwhich is incorporated herein by reference.

BACKGROUND

Embodiments of the subject matter disclosed herein relate to magneticresonance imaging (MRI), and more particularly, to MRI radio frequency(RF) coils.

Magnetic resonance imaging (MRI) is a medical imaging modality that cancreate images of the inside of a human body without using x-rays orother ionizing radiation. MRI systems include a superconducting magnetto create a strong, uniform, static magnetic field. When a human body,or part of a human body, is placed in the magnetic field, the nuclearspins associated with the hydrogen nuclei in tissue water becomepolarized, wherein the magnetic moments associated with these spinsbecome preferentially aligned along the direction of the magnetic field,resulting in a small net tissue magnetization along that axis. MRIsystems also include gradient coils that produce smaller amplitude,spatially-varying magnetic fields with orthogonal axes to spatiallyencode the magnetic resonance (MR) signal by creating a signatureresonance frequency at each location in the body. Radio frequency (RF)coils are then used to create pulses of RF energy at or near theresonance frequency of the hydrogen nuclei, which add energy to thenuclear spin system. As the nuclear spins relax back to their restenergy state, they release the absorbed energy in the form of an MRsignal. This signal is detected by the MRI system and is transformedinto an image using a computer and known reconstruction algorithms.

As mentioned, RF coils are used in MRI systems to transmit RF excitationsignals (“transmit coil”), and to receive the MR signals emitted by animaging subject (“receive coil”). Coil-interfacing cables may be used totransmit signals between the RF coils and other aspects of theprocessing system, for example to control the RF coils and/or to receiveinformation from the RF coils. However, conventional RF coils tend to bebulky, rigid and are configured to be maintained at a fixed positionrelative to other RF coils in an array. This bulkiness and lack offlexibility often prevents the RF coil loops from coupling mostefficiently with the desired anatomy and make them very uncomfortable tothe imaging subject. Further, coil-to-coil interactions dictate that thecoils be sized and/or positioned non-ideally from a coverage or imagingacceleration perspective.

SUMMARY

In one embodiment, an anterior radio frequency (RF) coil array for amagnetic resonance imaging (MRI) system includes a distributedcapacitance loop portion comprising two parallel wire conductorsencapsulated and separated by a dielectric material, the two parallelwire conductors maintained separate by the dielectric material along anentire length of the loop portion between terminating ends thereof, acoupling electronics portion including a pre-amplifier, and acoil-interfacing cable including a plurality of baluns or common modetraps positioned in a continuous and/or contiguous manner. In this way,a flexible RF coil assembly may be provided that allows for RF coils inan array to be positioned more arbitrarily, allowing placement and/orsize of the coils to be based on desired anatomy coverage, withouthaving to account for fixed coil overlaps or electronics positioning.The coils may conform to the subject anatomy with relative ease.Additionally, the cost and weight of the coils may be significantlylowered due to minimized materials and production process, andenvironmentally-friendlier processes may be used in the manufacture andminiaturization of the RF coils of the present disclosure versusconventional coils.

It should be understood that the brief description above is provided tointroduce in simplified form a selection of concepts that are furtherdescribed in the detailed description. It is not meant to identify keyor essential features of the claimed subject matter, the scope of whichis defined uniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will be better understood from reading thefollowing description of non-limiting embodiments, with reference to theattached drawings, wherein below:

FIG. 1 is a block diagram of an MRI system.

FIG. 2 schematically shows an example RF coil coupled to a controllerunit.

FIG. 3 shows a first example RF coil and associated couplingelectronics.

FIG. 4 shows a second example RF coil and associated couplingelectronics.

FIG. 5 shows a cross-sectional view of a distributed capacitance loopportion of an example RF coil.

FIG. 6 shows a block diagram of an example anterior RF coil array.

FIG. 7 schematically shows an example anterior RF coil array.

FIG. 8 shows an exploded view of an example anterior RF coil array.

FIG. 9 shows an example packaged anterior RF coil array.

FIG. 10 schematically shows an example RF coil array interfacing cableincluding a plurality of continuous and/or contiguous common mode trapspositioned between a processing system and a RF coil array of a MRIsystem.

FIGS. 11 and 12 schematically show example RF coil array interfacingcables including a plurality of continuous and/or contiguous common modetraps.

DETAILED DESCRIPTION

The following description relates to various embodiments of a radiofrequency (RF) coil in MRI systems. In particular, systems and methodsare provided for a low-cost, flexible, and lightweight RF coil that iseffectively transparent in multiple respects. The RF coil is effectivelytransparent to patients, given the low weight of the coil and flexiblepackaging that is enabled by the RF coil. The RF coil is alsoeffectively transparent to other RF coils in an array of RF coils, dueto minimization of magnetic and electric coupling mechanisms. Further,the RF coil is effectively transparent to other structures throughcapacitance minimization and is transparent to positrons through massreduction, enabling use of the RF coil in hybrid positron emissiontomography (PET)/MR imaging systems. The RF coil of the presentdisclosure may be used in MRI systems of various magnetic fieldstrengths.

The RF coil of the present disclosure includes a significantly smalleramount of copper, printed circuit board (PCB) material and electroniccomponents than used in a conventional RF coil and includes parallelelongated wire conductors, encapsulated and separated by a dielectricmaterial, forming the coil element. The parallel wires form a lowreactance structure without need for discrete capacitors. The minimalconductor, sized to keep losses tolerable, eliminates much of thecapacitance between coil loops, and reduces electric field coupling. Byinterfacing with a large sampling impedance, currents are reduced andmagnetic field coupling is minimized. The electronics are minimized insize and content to keep mass and weight low and prevent excessiveinteraction with the desired fields. Packaging can now be extremelyflexible which allows conforming to anatomy, optimizing signal to noiseratio (SNR) and imaging acceleration.

A traditional receive coil for MR is comprised of several conductiveintervals joined between themselves by capacitors. By adjusting thecapacitors' values, the impedance of the RF coil may be brought to itsminimal value, usually characterized by low resistance. At resonantfrequency, stored magnetic and electric energy alternate periodically.Each conductive interval, due to its length and width, possesses acertain self-capacitance, where electric energy is periodically storedas static electricity. The distribution of this electricity takes placeover the entire conductive interval length of the order of 5-15 cm,causing similar range electric dipole field. In a proximity of a largedielectric load, self-capacitance of the intervals change—hence detuningthe coil. In case of a lossy dielectric, dipole electric field causesJoule dissipation characterized by an increase overall resistanceobserved by the coil.

In contrast, the RF coil of the present disclosure represents almost anideal magnetic dipole antenna as its common mode current is uniform inphase and amplitude along its perimeter. The capacitance of the RF coilis built between the two wire conductors along the perimeter of theloop. The conservative electric field is strictly confined within thesmall cross-section of the two parallel wires and dielectric fillermaterial. In the case of two RF coils overlapping, the parasiticcapacitance at the cross-overs or overlaps is greatly reduced incomparison to two overlapped copper traces. RF coil thin cross-sectionsallows better magnetic decoupling and reduces or eliminates criticaloverlap between two loops in comparison to two traditional trace-basedloops.

FIG. 1 illustrates a magnetic resonance imaging (MRI) apparatus 10 thatincludes a superconducting magnet unit 12, a gradient coil unit 13, anRF coil unit 14, an RF body or volume coil unit 15, a transmit/receive(T/R) switch 20, an RF driver unit 22, a gradient coil driver unit 23, adata acquisition unit 24, a controller unit 25, a table 26, a dataprocessing unit 31, an operating console unit 32, and a display unit 33.In one example, the RF coil unit 14 is a surface coil, which is a localcoil that is typically placed proximate to the anatomy of interest of asubject 16. Herein, the RF body coil unit 15 is a transmit coil thattransmits RF signals, and the local surface RF coil unit 14 receives MRsignals. As such, the transmit body coil (e.g., RF body coil unit 15)and the surface receive coil (e.g., RF coil unit 14) are independent butelectromagnetically coupled structures. The MRI apparatus 10 transmitselectromagnetic pulse signals to the subject 16 placed in an imagingspace 18 with a static magnetic field formed to perform a scan forobtaining MR signals from the subject 16 to reconstruct an image of thesubject 16 based on the MR signals obtained by the scan.

The superconducting magnet unit 12 includes, for example, an annularsuperconducting magnet, which is mounted within a toroidal vacuumvessel. The magnet defines a cylindrical space surrounding the subject16, and generates a constant, strong, uniform, static magnetic fieldalong the Z direction of the cylindrical space.

The MRI apparatus 10 also includes the gradient coil unit 13 thatgenerates a gradient magnetic field in the imaging space 18 so as toprovide the MR signals received by the RF coil unit 14 withthree-dimensional positional information. The gradient coil unit 13includes three gradient coil systems, each of which generates a gradientmagnetic field, which inclines into one of three spatial axesperpendicular to each other, and generates a gradient magnetic field ineach of frequency encoding direction, phase encoding direction, andslice selection direction in accordance with the imaging condition. Morespecifically, the gradient coil unit 13 applies a gradient magneticfield in the slice selection direction of the subject 16, to select theslice; and the RF body coil unit 15 transmits an RF signal to a selectedregion of interest (ROI) of the subject 16 and excites it. The gradientcoil unit 13 also applies a gradient magnetic field in the phaseencoding direction of the subject 16 to phase encode the MR signals fromthe ROI excited by the RF signal. The gradient coil unit 13 then appliesa gradient magnetic field in the frequency encoding direction of thesubject 16 to frequency encode the MR signals from the ROI excited bythe RF signal.

The RF coil unit 14 is disposed, for example, to enclose the region tobe imaged of the subject 16. In some examples, the RF coil unit 14 maybe referred to as the surface coil or the receive coil. In the staticmagnetic field space or imaging space 18 where a static magnetic fieldis formed by the superconducting magnet unit 12, the RF coil unit 14transmits, based on a control signal from the controller unit 25, an RFsignal that is an electromagnet wave to the subject 16 and therebygenerates a high-frequency magnetic field. This excites a spin ofprotons in the slice to be imaged of the subject 16. The RF coil unit 14receives, as a magnetic resonance signal, the electromagnetic wavegenerated when the proton spin thus excited in the slice to be imaged ofthe subject 16 returns into alignment with the initial magnetizationvector. The RF coil unit 14 may transmit and receive an RF signal usingthe same RF coil.

The RF body coil unit 15 is disposed, for example, to enclose theimaging space 18, and produces RF magnetic field pulses orthogonal tothe main magnetic field produced by the superconducting magnet unit 12within the imaging space 18 to excite the nuclei. In contrast to the RFcoil unit 14, which may be disconnected from the MRI apparatus 10 andreplaced with another RF coil unit, the RF body coil unit 15 is fixedlyattached and connected to the MRI apparatus 10. Furthermore, whereaslocal coils such as those comprising the RF coil unit 14 can transmit toor receive signals from only a localized region of the subject 16, theRF body coil unit 15 generally have a larger coverage area. The RF bodycoil unit 15 may be used to transmit or receive signals to the wholebody of the subject 16, for example. Using receive-only local coils andtransmit body coils provides a uniform RF excitation and good imageuniformity at the expense of high RF power deposited in the subject. Fora transmit-receive local coil, the local coil provides the RF excitationto the region of interest and receives the MR signal, thereby decreasingthe RF power deposited in the subject. It should be appreciated that theparticular use of the RF coil unit 14 and/or the RF body coil unit 15depends on the imaging application.

The T/R switch 20 can selectively electrically connect the RF body coilunit 15 to the data acquisition unit 24 when operating in receive mode,and to the RF driver unit 22 when operating in transmit mode. Similarly,the T/R switch 20 can selectively electrically connect the RF coil unit14 to the data acquisition unit 24 when the RF coil unit 14 operates inreceive mode, and to the RF driver unit 22 when operating in transmitmode. When the RF coil unit 14 and the RF body coil unit 15 are bothused in a single scan, for example if the RF coil unit 14 is configuredto receive MR signals and the RF body coil unit 15 is configured totransmit RF signals, then the T/R switch 20 may direct control signalsfrom the RF driver unit 22 to the RF body coil unit 15 while directingreceived MR signals from the RF coil unit 14 to the data acquisitionunit 24. The coils of the RF body coil unit 15 may be configured tooperate in a transmit-only mode, a receive-only mode, or atransmit-receive mode. The coils of the local RF coil unit 14 may beconfigured to operate in a transmit-receive mode or a receive-only mode.

The RF driver unit 22 includes a gate modulator (not shown), an RF poweramplifier (not shown), and an RF oscillator (not shown) that are used todrive the RF coil unit 14 and form a high-frequency magnetic field inthe imaging space 18. The RF driver unit 22 modulates, based on acontrol signal from the controller unit 25 and using the gate modulator,the RF signal received from the RF oscillator into a signal ofpredetermined timing having a predetermined envelope. The RF signalmodulated by the gate modulator is amplified by the RF power amplifierand then output to the RF coil unit 14.

The gradient coil driver unit 23 drives the gradient coil unit 13 basedon a control signal from the controller unit 25 and thereby generates agradient magnetic field in the imaging space 18. The gradient coildriver unit 23 includes three systems of driver circuits (not shown)corresponding to the three gradient coil systems included in thegradient coil unit 13.

The data acquisition unit 24 includes a pre-amplifier (not shown), aphase detector (not shown), and an analog/digital converter (not shown)used to acquire the magnetic resonance signals received by the RF coilunit 14. In the data acquisition unit 24, the phase detector phasedetects, using the output from the RF oscillator of the RF driver unit22 as a reference signal, the magnetic resonance signals received fromthe RF coil unit 14 and amplified by the pre-amplifier, and outputs thephase-detected analog magnetic resonance signals to the analog/digitalconverter for conversion into digital signals. The digital signals thusobtained are output to the data processing unit 31.

The MRI apparatus 10 includes a table 26 for placing the subject 16thereon. The subject 16 may be moved inside and outside the imagingspace 18 by moving the table 26 based on control signals from thecontroller unit 25.

The controller unit 25 includes a computer and a recording medium onwhich a program to be executed by the computer is recorded. The programwhen executed by the computer causes various parts of the apparatus tocarry out operations corresponding to predetermined scanning. Therecording medium may comprise, for example, a ROM, flexible disk, harddisk, optical disk, magneto-optical disk, CD-ROM, or non-volatilememory. The controller unit 25 is connected to the operating consoleunit 32 and processes the operation signals input to the operatingconsole unit 32 and furthermore controls the table 26, RF driver unit22, gradient coil driver unit 23, and data acquisition unit 24 byoutputting control signals to them. The controller unit 25 alsocontrols, to obtain a desired image, the data processing unit 31 and thedisplay unit 33 based on operation signals received from the operatingconsole unit 32.

The operating console unit 32 includes user input devices such as atouchscreen, keyboard and a mouse. The operating console unit 32 is usedby an operator, for example, to input such data as an imaging protocoland to set a region where an imaging sequence is to be executed. Thedata about the imaging protocol and the imaging sequence executionregion are output to the controller unit 25.

The data processing unit 31 includes a computer and a recording mediumon which a program to be executed by the computer to performpredetermined data processing is recorded. The data processing unit 31is connected to the controller unit 25 and performs data processingbased on control signals received from the controller unit 25. The dataprocessing unit 31 is also connected to the data acquisition unit 24 andgenerates spectrum data by applying various image processing operationsto the magnetic resonance signals output from the data acquisition unit24.

The display unit 33 includes a display device and displays an image onthe display screen of the display device based on control signalsreceived from the controller unit 25. The display unit 33 displays, forexample, an image regarding an input item about which the operatorinputs operation data from the operating console unit 32. The displayunit 33 also displays a two-dimensional (2D) slice image orthree-dimensional (3D) image of the subject 16 generated by the dataprocessing unit 31.

During a scan, RF coil array interfacing cables (not shown) may be usedto transmit signals between the RF coils (e.g., RF coil unit 14 and RFbody coil unit 15) and other aspects of the processing system (e.g.,data acquisition unit 24, controller unit 25, and so on), for example tocontrol the RF coils and/or to receive information from the RF coils. Asexplained previously, the RF body coil unit 15 is a transmit coil thattransmits RF signals, and the local surface RF coil unit 14 receives theMR signals. More generally, RF coils are used to transmit RF excitationsignals (“transmit coil”), and to receive the MR signals emitted by animaging subject (“receive coil”). In an example, the transmit andreceive coils are a single mechanical and electrical structure or arrayof structures, with transmit/receive mode switchable by auxiliarycircuitry. In other examples, the transmit body coil (e.g., RF body coilunit 15) and the surface receive coil (e.g., RF coil unit 14) may beindependent structures that are physically coupled to each other via adata acquisition unit or other processing unit. For enhanced imagequality, however, it may be desirable to provide a receive coil that ismechanically and electrically isolated from the transmit coil. In suchcase it is desirable that the receive coil, in its receive mode, beelectromagnetically coupled to and resonant with an RF “echo” that isstimulated by the transmit coil. However, during transmit mode, it maybe desirable that the receive coil is electromagnetically decoupled fromand therefore not resonant with the transmit coil, during actualtransmission of the RF signal. Such decoupling averts a potentialproblem of noise produced within the auxiliary circuitry when thereceive coil couples to the full power of the RF signal. Additionaldetails regarding the uncoupling of the receive RF coil will bedescribed below.

As mentioned previously, traditional RF coils may include acid etchedcopper traces (loops) on PCBs with lumped electronic components (e.g.,capacitors, inductors, baluns, resisters, etc.), matching circuitry,decoupling circuitry, and pre-amplifiers. Such a configuration istypically very bulky, heavy and rigid, and requires relatively strictplacement of the coils relative to each other in an array to preventcoupling interactions among coil elements that may degrade imagequality. As such, traditional RF coils and RF coil arrays lackflexibility and hence may not conform to subject anatomy, degradingimaging quality and patient comfort.

Thus, according to embodiments disclosed herein, an RF coil array, suchas RF coil unit 14, may include distributed capacitance wires ratherthan copper traces on PCBs with lumped electronic components. As aresult, the RF coil array may be lightweight and flexible, allowingplacement in low-cost, lightweight, waterproof, and/or flame-retardantfabrics or materials. The coupling electronics portion coupling the loopportion of the RF coil (e.g., the distributed capacitance wire) may beminiaturized and utilize a low input impedance pre-amplifier, which isoptimized for high source impedance (e.g., due to impedance matchingcircuitry) and allows flexible overlaps among coil elements in an RFcoil array. Further, the RF coil array interfacing cable between the RFcoil array and system processing components may be flexible and includeintegrated transparency functionality in the form of distributed baluns,which allows rigid electronic components to be avoided and aids inspreading of the heat load.

Turning now to FIG. 2, a schematic view of an RF coil 202 including aloop portion 201 coupled to a controller unit 210 via a couplingelectronics portion 203 and a coil-interfacing cable 212 is shown. Inone example, the RF coil may be a surface receive coil, which may besingle- or multi-channel. The RF coil 202 is one non-limiting example ofRF coil unit 14 of FIG. 1 and as such may operate at one or morefrequencies in the MRI apparatus 10. The coil-interfacing cable 212 maybe a coil-interfacing cable extending between the electronics portion203 and an interfacing connector of an RF coil array or a RF coil arrayinterfacing cable extending between the interfacing connector of the RFcoil array and the MRI system controller unit 210. The controller unit210 may be associated with and/or may be a non-limiting example of thedata processing unit 31 or controller unit 25 in FIG. 1.

The coupling electronics portion 203 may be coupled to the loop portionof the RF coil 202. Herein, the coupling electronics portion 203 mayinclude a decoupling circuit 204, impedance inverter circuit 206, and apre-amplifier 208. The decoupling circuit 204 may effectively decouplethe RF coil during a transmit operation. Typically, the RF coil 202 inits receive mode may be coupled to a body of a subject being imaged bythe MR apparatus in order to receive echoes of the RF signal transmittedduring the transmit mode. If the RF coil 202 is not used fortransmission, then it may be necessary to decouple the RF coil 202 fromthe RF body coil while the RF body coil is transmitting the RF signal.The decoupling of the receive coil from the transmit coil may beachieved using resonance circuits and PIN diodes, microelectromechanicalsystems (MEMS) switches, or another type of switching circuitry. Herein,the switching circuitry may activate detuning circuits operativelyconnected to the RF coil 202.

The impedance inverter circuit 206 may form an impedance matchingnetwork between the RF coil 202 and the pre-amplifier 208. The impedanceinverter circuit 206 is configured to transform a coil impedance of theRF coil 202 into an optimal source impedance for the pre-amplifier 208.The impedance inverter circuit 206 may include an impedance matchingnetwork and an input balun. The pre-amplifier 208 receives MR signalsfrom the corresponding RF coil 202 and amplifies the received MRsignals. In one example, the pre-amplifier may have a low inputimpedance that is configured to accommodate a relatively high blockingor source impedance. Additional details regarding the RF coil andassociated coupling electronics portion will be explained in more detailbelow with respect to FIGS. 3 and 4. The coupling electronics portion203 may be packaged in a very small PCB approximately 2 cm² in size orsmaller. The PCB may be protected with a conformal coating or anencapsulating resin.

The coil-interfacing cable 212, such as a RF coil array interfacingcable, may be used to transmit signals between the RF coils and otheraspects of the processing system, for example to control the RF coilsand/or to receive information from the RF coils. The RF coil arrayinterfacing cables may be disposed within the bore or imaging space ofthe MRI apparatus (such as MRI apparatus 10 of FIG. 1) and subjected toelectro-magnetic fields produced and used by the MRI apparatus. In MRIsystems, coil-interfacing cables, such as coil-interfacing cable 212,may support transmitter-driven common-mode currents, which may in turncreate field distortions and/or unpredictable heating of components.Typically, common-mode currents are blocked by using baluns. Baluns orcommon-mode traps provide high common-mode impedances, which in turnreduces the effect of transmitter-driven currents.

Thus, coil-interfacing cable 212 may include one or more baluns. Intraditional coil-interfacing cables, baluns are positioned with arelatively high density, as high dissipation/voltages may develop if thebalun density is too low or if baluns are positioned at an inappropriatelocation. However, this dense placement may adversely affectflexibility, cost, and performance. As such, the one or more baluns inthe coil-interfacing cable may be continuous baluns to ensure no highcurrents or standing waves, independent of positioning. The continuousbaluns may be distributed, flutter, and/or butterfly baluns. Additionaldetails regarding the coil-interfacing cable and baluns will bepresented below with respect to FIGS. 10, 11 and 12.

FIG. 3 is a schematic of an RF coil 301 having segmented conductorsformed in accordance with an embodiment. RF coil 301 is a non-limitingexample of RF coil 202 of FIG. 2 and as such includes loop portion 201and coupling electronics portion 203 of RF coil 202. The couplingelectronics portion allows the RF coil to transmit and/or receive RFsignals when driven by the data acquisition unit 124 (shown in FIG. 1).In the illustrated embodiment, the RF coil 301 includes a firstconductor 300 and a second conductor 302. The first and secondconductors 300, 302 may be segmented such that the conductors form anopen circuit (e.g., form a monopole). The segments of the conductors300, 302 may have different lengths, as is discussed below. The lengthof the first and second conductors 300, 302 may be varied to achieve aselect distributed capacitance, and accordingly, a select resonancefrequency.

The first conductor 300 includes a first segment 304 and a secondsegment 306. The first segment 304 includes a driven end 312 at aninterface terminating to coupling electronics portion 203, which will bedescribed in more detail below. The first segment 304 also includes afloating end 314 that is detached from a reference ground, therebymaintaining a floating state. The second segment 306 includes a drivenend 316 at the interface terminating to the coupling electronics portionand a floating end 318 that is detached from a reference ground.

The second conductor 302 includes a first segment 308 and a secondsegment 310. The first segment 308 includes a driven end 320 at theinterface. The first segment 308 also includes a floating end 322 thatis detached from a reference ground, thereby maintaining a floatingstate. The second segment 310 includes a driven end 324 at theinterface, and a floating end 326 that is detached from a referenceground. The driven end 324 may terminate at the interface such that end324 is only coupled to the first conductor through the distributedcapacitance. The capacitors shown around the loop between the conductorsrepresent the capacitance between the wire conductors.

The first conductor 300 exhibits a distributed capacitance that growsbased on the length of the first and second segments 304, 306. Thesecond conductor 302 exhibits a distributed capacitance that grows basedon the length of the first and second segments 308, 310. The firstsegments 304, 308 may have a different length than the second segments306, 310. The relative difference in length between the first segments304, 308 and the second segments 306, 310 may be used to produce aneffective LC circuit have a resonance frequency at the desired centerfrequency. For example, by varying the length of the first segments 304,308 relative to the lengths of the second segments 306, 310, anintegrated distributed capacitance may be varied.

In the illustrated embodiment, the first and second wire conductors 300,302 are shaped into a loop portion that terminates to an interface. Butin other embodiments, other shapes are possible. For example, the loopportion may be a polygon, shaped to conform the contours of a surface(e.g., housing), and/or the like. The loop portion defines a conductivepathway along the first and second conductors. The first and secondconductors are void of any discrete or lumped capacitive or inductiveelements along an entire length of the conductive pathway. The loopportion may also include loops of varying gauge of stranded or solidconductor wire, loops of varying diameters with varying lengths of thefirst and second conductors 300, 302, and/or loops of varying spacingbetween the first and second conductors. For example, each of the firstand second conductors may have no cuts or gaps (no segmented conductors)or one or more cuts or gaps (segmented conductors) at various locationsalong the conductive pathway.

Distributed capacitance (DCAP), as used herein, represents a capacitanceexhibited between conductors that grows evenly and uniformly along thelength of the conductors and is void of discrete or lumped capacitivecomponents and discrete or lumped inductive components. In the examplesherein, the capacitance may grow in a uniform manner along the length ofthe first and second conductors 300, 302.

A dielectric material 303 encapsulates and separates the first andsecond conductors 300, 302. The dielectric material 303 may beselectively chosen to achieve a select distributive capacitance. Thedielectric material 303 may be based on a desired permittivity E to varythe effective capacitance of the loop portion. For example, thedielectric material 303 may be air, rubber, plastic, or any otherdielectric material. In one example, the dielectric material may bepolytetrafluoroethylene (pTFE). For example, the dielectric material 303may be an insulating material surrounding the parallel conductiveelements of the first and second conductors 300, 302. Alternatively, thefirst and second conductors 300, 302 may be twisted upon one another tofrom a twisted pair cable. As another example, the dielectric material303 may be a plastic material. The first and second conductors 300, 302may form a coaxial structure in which the plastic dielectric material303 separates the first and second conductors. As another example, thefirst and second conductors may be configured as planar strips.

The coupling electronics portion 203 is operably and communicativelycoupled to the RF driver unit 22, the data acquisition unit 24,controller unit 25, and/or data processing unit 31 to allow the RF coil102 to transmit and/or receive RF signals. In the illustratedembodiment, the coupling electronics portion 203 includes a signalinterface 358 configured to transmit and receive the RF signals. Thesignal interface 358 may transmit and receive the RF signals via acable. The cable may be a 3-conductor triaxial cable having a centerconductor, an inner shield, and an outer shield. The center conductor isconnected to the RF signal and pre-amp control (RF), the inner shield isconnected to ground (GND), and the outer shield is connected to themulti-control bias (diode decoupling control) (MC_BIAS). A 10V powerconnection may be carried on the same conductor as the RF signal.

As explained above with respect to FIG. 2, the coupling electronicsportion 203 includes a decoupling circuit, impedance inverter circuit,and pre-amplifier. As illustrated in FIG. 3, the decoupling circuitincludes a decoupling diode 360. The decoupling diode 360 may beprovided with voltage from MC_BIAS, for example, in order to turndecoupling diode 360 on. When on, decoupling diode 360 causes conductor300 to short with conductor 302, thus causing the coil be off-resonanceand hence decouple the coil during a transmit operation, for example.

The impedance inverter circuit includes a plurality of inductors,including first inductor 370 a, second inductor 370 b, and thirdinductor 370 c; a plurality of capacitors, including first capacitor 372a, a second capacitor 372 b, a third capacitor 372 c, and a fourthcapacitor 372 d; and a diode 374. The impedance inverter circuitincludes matching circuitry and an input balun. As shown, the inputbalun is a lattice balun that comprises first inductor 370 a, secondinductor 370 b, first capacitor 372 a, and second capacitor 372 b. Inone example, diode 374 limits the direction of current flow to block RFreceive signals from proceeding into decoupling bias branch (MC_BIAS).

The pre-amplifier 362 may be a low input impedance pre-amplifier that isoptimized for high source impedance by the impedance matching circuitry.The pre-amplifier may have a low noise reflection coefficient, γ, and alow noise resistance, Rn. In one example, the pre-amplifier may have asource reflection coefficient of γ substantially equal to 0.0 and anormalized noise resistance of Rn substantially equal to 0.0 in additionto the low noise figure. However, γ values substantially equal to orless than 0.1 and Rn values substantially equal to or less than 0.2 arealso contemplated. With the pre-amplifier having the appropriate γ andRn values, the pre-amplifier provides a blocking impedance for RF coil301 while also providing a large noise circle in the context of a SmithChart. As such, current in RF coil 301 is minimized, the pre-amplifieris effectively noise matched with RF coil 301 output impedance. Having alarge noise circle, the pre-amplifier yields an effective SNR over avariety of RF coil impedances while producing a high blocking impedanceto RF coil 301.

In some examples, the pre-amplifier 362 may include an impedancetransformer that includes a capacitor and an inductor. The impedancetransformer may be configured to alter the impedance of thepre-amplifier to effectively cancel out a reactance of thepre-amplifier, such as capacitance caused by a parasitic capacitanceeffect. Parasitic capacitance effects can be caused by, for example, aPCB layout of the pre-amplifier or by a gate of the pre-amplifier.Further, such reactance can often increase as the frequency increases.Advantageously, however, configuring the impedance transformer of thepre-amplifier to cancel, or at least minimize, reactance maintains ahigh impedance (i.e. a blocking impedance) to RF coil 301 and aneffective SNR without having a substantial impact on the noise figure ofthe pre-amplifier. The lattice balun described above may be anon-limiting example of an impedance transformer.

In examples, the pre-amplifier described herein may a low inputpre-amplifier. For example, in some embodiments, a “relatively low”input impedance of the preamplifier is less than approximately 5 ohms atresonance frequency. The coil impedance of the RF coil 301 may have anyvalue, which may be dependent on coil loading, coil size, fieldstrength, and/or the like. Examples of the coil impedance of the RF coil301 include, but are not limited to, between approximately 2 ohms andapproximately 10 ohms at 1.5 T magnetic field strength, and/or the like.The impedance inverter circuitry is configured to transform the coilimpedance of the RF coil 301 into a relatively high source impedance.For example, in some embodiments, a “relatively high” source impedanceis at least approximately 100 ohms and may be greater than 150 ohms.

The impedance transformer may also provide a blocking impedance to theRF coil 301. Transformation of the coil impedance of the RF coil 301 toa relative high source impedance may enable the impedance transformer toprovide a higher blocking impedance to the RF coil 301. Exemplary valuesfor such higher blocking impedances include, for example, a blockingimpedance of at least 500 ohms, and at least 1000 ohms.

FIG. 4 is a schematic of an RF coil 401 and coupling electronics portion203 according to another embodiment. The RF coil of FIG. 4 is anon-limiting example of the RF coil and coupling electronics of FIG. 2,and as such includes a loop portion 201 and coupling electronics portion203. The coupling electronics allows the RF coil to transmit and/orreceive RF signals when driven by the data acquisition unit 124 (shownin FIG. 1). The RF coil 401 includes a first conductor 400 in parallelwith a second conductor 402. At least one of the first and secondconductors 400, 402 are elongated and continuous.

In the illustrated embodiment, the first and second conductors 400, 402are shaped into a loop portion that terminates to an interface. But inother embodiments, other shapes are possible. For example, the loopportion may be a polygon, shaped to conform the contours of a surface(e.g., housing), and/or the like. The loop portion defines a conductivepathway along the first and second conductors 400, 402. The first andsecond conductors 400, 402 are void of any discrete or lumped capacitiveor inductive components along an entire length of the conductivepathway. The first and second conductors 400, 402 are uninterrupted andcontinuous along an entire length of the loop portion. The loop portionmay also include loops of varying gauge of stranded or solid conductorwire, loops of varying diameters with varying lengths of the first andsecond conductors 400, 402, and/or loops of varying spacing between thefirst and second conductors. For example, each of the first and secondconductors may have no cuts or gaps (no segmented conductors) or one ormore cuts or gaps (segmented conductors) at various locations along theconductive pathway.

The first and second conductors 400, 402 have a distributed capacitancealong the length of the loop portion (e.g., along the length of thefirst and second conductors 400, 402). The first and second conductors400, 402 exhibit a substantially equal and uniform capacitance along theentire length of the loop portion. Distributed capacitance (DCAP), asused herein, represents a capacitance exhibited between conductors thatgrows evenly and uniformly along the length of the conductors and isvoid of discrete or lumped capacitive components and discrete or lumpedinductive components. In the examples herein, the capacitance may growin a uniform manner along the length of the first and second conductors400, 402. At least one of the first and second conductors 400, 402 areelongated and continuous. In the illustrated embodiment, both the firstand second conductors 400, 402 are elongated and continuous. But inother embodiments, only one of the first or second conductors 400, 402may be elongated and continuous. The first and second conductors 400,402 form continuous distributed capacitors. The capacitance grows at asubstantially constant rate along the length of the conductors 400, 402.In the illustrated embodiment, the first and second conductors 400, 402form elongated continuous conductors that exhibits DCAP along the lengthof the first and second conductors 400, 402. The first and secondconductors 400, 402 are void of any discrete capacitive and inductivecomponents along the entire length of the continuous conductors betweenterminating ends of the first and second conductors 400, 402. Forexample, the first and second conductors 400, 402 do not include anydiscrete capacitors, nor any inductors along the length of the loopportion.

A dielectric material 403 separates the first and second conductors 400,402. The dielectric material 403 may be selectively chosen to achieve aselect distributive capacitance. The dielectric material 403 may bebased on a desired permittivity E to vary the effective capacitance ofthe loop portion. For example, the dielectric material 403 may be air,rubber, plastic, or any other dielectric material. In one example, thedielectric material may be polytetrafluoroethylene (pTFB). For example,the dielectric material 403 may be an insulating material surroundingthe parallel conductive elements of the first and second conductors 400,402. Alternatively, the first and second conductors 400, 402 may betwisted upon one another to from a twisted pair cable. As anotherexample, the dielectric material 403 may be a plastic material. Thefirst and second conductors 400, 402 may form a coaxial structure inwhich the plastic dielectric material 403 separates the first and secondconductors 400, 402. As another example, the first and second conductors400, 402 may be configured as planar strips.

The first conductor 400 includes a first terminating end 412 and asecond terminating end 416 that terminates at the interface. The firstterminating end 412 is coupled to the coupling electronics portion 203.The first terminating end 412 may also be referred to herein as a “driveend.” The second terminating end 416 is also referred to herein as a“second drive end.”

The second conductor 402 includes a first terminating end 420 and asecond terminating end 424 that terminates at the interface. The firstterminating end 420 is coupled to the coupling electronics portion 203.The first terminating end 420 may also be referred to herein as a “driveend.” The second terminating end 424 is also referred to herein as a“second drive end.”

The loop portion 201 of the RF coil 401 is coupled to couplingelectronics portion 203. The coupling electronics portion 203 may be thesame coupling electronics described above with respect to FIGS. 2 and 3,and hence like reference numbers are given to like components andfurther description is dispensed with.

As appreciated by FIGS. 3 and 4, the two parallel conductors comprisingthe loop portion of an RF coil may each be continuous conductors, asillustrated in FIG. 4, or one or both of the conductors may benon-continuous, as illustrated in FIG. 3. For example, both conductorsshown in FIG. 3 may include cuts, resulting in each conductor beingcomprised of two segments. The resulting space between conductorsegments may be filled with the dielectric material that encapsulatesand surrounds the conductors. The two cuts may be positioned atdifferent locations, e.g., one cut at 135° and the other cut at 225°(relative to where the loop portion interfaces with the couplingelectronics). By including discontinuous conductors, the resonancefrequency of the coil may be adjusted relative to a coil that includescontinuous conductors. In an example, an RF coil that includes twocontinuous parallel conductors encapsulated and separated by adielectric, the resonance frequency may be a smaller, first resonancefrequency. If that RF coil instead includes one discontinuous conductor(e.g., where one of the conductors is cut and filled with the dielectricmaterial) and one continuous conductor, with all other parameters (e.g.,conductor wire gauge, loop diameter, spacing between conductors,dielectric material) being the same, the resonance frequency of the RFcoil may be a larger, second resonance frequency. In this way,parameters of the loop portion, including conductor wire gauge, loopdiameter, spacing between conductors, dielectric material selectionand/or thickness, and conductor segment number and lengths, may beadjusted to tune the RF coil to a desired resonance frequency.

FIG. 5 shows a cross-sectional view of a distributed capacitance loopportion 500 of an example RF coil. As appreciated by FIG. 5, loopportion 500 includes first wire conductor 502 and second wire conductor504 surrounded by and encapsulated in dielectric material 503. Each wireconductor may have a suitable cross-sectional shape, herein a circularcross-sectional shape. However, other cross-sectional shapes for thewire conductors are possible, such as elliptical, cylindrical,rectangular, triangular, hexagonal, etc. The wire conductors may beseparated by a suitable distance, and the distance separating theconductors as well as the diameters of the wire conductors may beselected to achieve a desired capacitance. Further, each of the firstwire conductor 502 and second wire conductor 504 may be a sevenconductor stranded wire (e.g., comprised of seven stranded wires), butsolid conductors may also be used instead of stranded wire. Strandedwire may provide more flexibility relative to solid conductors, at leastin some examples.

The RF coils presented above with respect to FIGS. 2-5 may be utilizedin order to receive MR signals during an MR imaging session. As such,the RF coils of FIGS. 2-5 may be non-limiting examples of RF coil unit14 of FIG. 1 and may be configured to be coupled to a downstreamcomponent of the MRI system, such as a processing system. The RF coilsof FIGS. 2-5 may be present in an array of RF coils having variousconfigurations. FIGS. 6-9, described in more detail below, illustratevarious embodiments for an anterior RF coil array that may include oneor more of the RF coils described above with respect to FIGS. 2-5. Theanterior RF coil array may be a high density (referring to the number ofcoil elements) anterior array or a high definition (referring to imageresolution) anterior array (HDAA).

FIG. 6 shows a block diagram of an example anterior RF coil array 600.The RF coil array 600 is a 30-channel high HDAA. 30-channel means thatthere are six rows of RF coils with each row comprising five RF coils.The RF coil array 600 includes 30 RF coil loops 601 arranged in a sixrow by five column array. Each of the RF coil loops 601 is coupled to acoupling electronics unit or printed circuit board (PCB) 603. Acoil-interfacing cable 612 is connected to and extends from eachcoupling electronics PCB 603 to an interfacing connector 620.

The coupling electronics unit 603 may include a decoupling circuit,impedance inverter circuit, and a pre-amplifier. The decoupling circuitmay effectively decouple an RF coil during a transmit operation. Theimpedance inverter circuit may form an impedance matching networkbetween an RF coil and the pre-amplifier. The impedance inverter circuitis configured to transform a coil impedance of a RF coil into an optimalsource impedance for the pre-amplifier. The impedance inverter circuitmay include an impedance matching network and an input balun. Thepre-amplifier receives MR signals from a RF coil and amplifies thereceived MR signals. In one example, the pre-amplifier may have a lowinput impedance that is configured to accommodate a relatively highblocking or source impedance. The coupling electronics unit 603 may bepackaged in a very small PCB approximately 2 cm² in size or smaller. ThePCB may be protected with a conformal coating or an encapsulating resin.

Control circuitry 615 is the MC_BIAS to switch RF coils between receiveand decoupled modes. Elements of the control circuitry 615 areincorporated in both the coupling electronics unit 603 and theinterfacing connector 620.

In one example, the anterior RF coil array 600 is a surface receivecoil, which may be designed for use with a 3 Tesla or 1.5 Tesla MRIsystem. When used with other coils, the indications for use includechest, cardiac, abdomen, torso, pelvis, prostate, hips, peripheralvascular, long bone and whole-body imaging.

FIG. 7 illustrates an example RF coil array 700 that may be implementedas a HDAA. The illustrated RF coil array 700 includes six rows of RFcoils with each row comprising five RF coils. Each RF coil including aRF coil loop 701 with a coupling electronics PCB 703 coupled to each RFcoil loop 701. The RF coil loops 701 are arranged such that the couplingelectronics PCBs 703 are all oriented towards the center of the RF coilarray, so that the coil-interfacing cables extending from each couplingelectronics PCB 703 extend down toward the center of the array. There isa coil-interfacing cable 712 extending from each coupling electronicsPCB. Each of these coil-interfacing cables are bundled together to forma cable assembly 760 including baluns. The cable assembly connects to asingle interfacing connector 720 at the end of the HDAA coil array tointerface with a RF coil array interfacing cable.

FIG. 7 shows an example RF coil array 700 including 30 RF coils 701attached to a fabric support 740, such as a sheet of flexible material.Each RF coil of the array is a non-limiting example of the RF coil loopsdescribed above with respect to FIGS. 2-5 and as such includes anintegrated capacitor coil loop 701 and a coupling electronics unit 703directly coupled to the coil loop. In some examples, the coilinterfacing cables and/or cable assembly of the array may includecontinuous/contiguous baluns throughout their length to eliminate thecylinder-shaped lumpy baluns. Thus, even when stitched or otherwiseattached to a support 740, each RF coil maintains flexibility inmultiple dimensions.

Thus, RF coil array 700 includes six rows of RF coils 701, including afirst row 710, second row 706, third row 711, fourth row 716, fifth row721, and sixth row 726. Each row includes five RF coils and associatedcoupling electronics. The RF coil and coupling electronics arrangementof first row 710 will be described in more detail below, but it is to beunderstood that each other row includes a similar configuration andhence additional description of each row is dispensed with.

First row 710 includes five RF coils, and each RF coil includes a loopportion comprised of distributed capacitance parallel wire conductorsand a coupling electronics unit in the form of a miniaturized PCBsupporting decoupling circuitry, impedance matching circuitry, and apre-amplifier. Each RF coil loop and coupling electronics unit may be anon-limiting example of the loop portion and coupling electronicsportion described above with respect to FIGS. 2 and 3.

Accordingly, a loop portion of a first RF coil 702 is coupled to a firstcoupling electronics unit 704, a loop portion of a second RF coil iscoupled to a second coupling electronics unit, a loop portion of a thirdRF coil is coupled to a third coupling electronics unit, a loop portionof a fourth RF coil is coupled to a fourth coupling electronics unit,and a loop portion of a fifth RF coil is coupled to a fifth couplingelectronics unit.

Each coupling electronics unit may be coupled to its respective RF coilloop in such a manner that each coupling electronics unit is orientatedtoward the center of the RF coil array 700. For example, first couplingelectronics unit 704 and second coupling electronics unit may each becoupled to a right side of a respective RF coil loop, fourth couplingelectronics unit and fifth coupling electronics unit may each be coupledto a left side of a respective RF coil loop, and third couplingelectronics unit may be coupled to a bottom side of its RF coil loop.

By orientating each coupling electronics unit toward the center of thearray, the coil-interfacing cable from each coupling electronics unit tothe cable assembly 760 and ultimately the RF coil array interfacingcable may be simplified. As shown in FIG. 7, each coupling electronicsunit 703 includes a coil-interfacing cable 712 extending therefrom tothe cable assembly 760 and to an interfacing connector 720. Eachcoil-interfacing cable of the first row joins into the cable assemblythat then runs along a central axis 750 of the RF coil array to theinterfacing connector.

A plurality of baluns 730 may be distributed along the coil interfacingcables and the cable assembly 760. As shown, there are at least twobaluns per row and a plurality of baluns along the cable assembly.However, other numbers and/or configurations of baluns are possible. Forexample, rather than including lumped baluns, each coil-interfacingcable (or cable assembly) may be encased in a continguous/continuousdistributed or flutter balun, such as those described with respect toFIGS. 10-12.

The coil-interfacing cables may include one or more baluns. Intraditional coil-interfacing cables, baluns are positioned with arelatively high density, as high dissipation/voltages may develop if thebalun density is too low or if baluns are positioned at an inappropriatelocation. However, this dense placement may adversely affectflexibility, cost, and performance. As such, the one or more baluns inthe coil-interfacing cable may be continuous baluns to ensure no highcurrents or standing waves, independent of positioning. The continuousbaluns may be distributed, flutter, and/or butterfly baluns. Additionaldetails regarding the coil-interfacing cable and baluns will bepresented below with respect to FIGS. 10-12.

The coil-interfacing cables 712 coupled to the coupling electronicsunits 703 for each row may combine into a central cable assembly 760 asdescribed above. The coil-interfacing cables 712 from the plurality ofRF coils 701 forming a cable assembly 760. The cable assembly 760extending along a central axis 750 of the RF coil array 700 to theinterfacing connector 720.

FIG. 8 shows an exploded view of an example anterior (HDAA) RF coilarray 800. The RF coil array 800 is attached or stitched to a flexiblesupport material, such as Norfab cloth as shown in layer 802. Each RFcoil loop of the RF coil array is coupled to the flexible supportmaterial, via stitching or other attachment mechanism. Sandwiching theRF coil array and attached flexible support material is an innerenclosure comprising a first layer 804 and second layer 806 of material.The material of the inner enclosure 804, 806 may be NOMEX® felt or othersuitable material that provides padding, spacing, and/or flame-retardantproperties. An optional inner layer of material 805 that is die cut withcutouts for the coupling electronics PCBs and baluns may be includedbetween the first layer 804 and the RF coil array attached to theflexible support material 802. The material of the optional inner layerof material 805 may be NOMEX® felt or other suitable material thatprovides padding, spacing, and/or flame-retardant properties. An outerenclosure comprising a first layer 810 and a second layer 812 ofmaterial sandwiches the coil array, attached support material, and innerenclosure. The material of the outer enclosure 810, 812 may be comprisedof a biocompatible material that is cleanable, thus enabling use of theRF coil array in clinical contexts, such as DARTEX®.

One example clinical context that the RF coils of the present disclosuremay be implemented is a high density/high definition anterior array(HDAA) used to cover the chest and/or abdomen of a subject undergoing MRimaging. FIG. 9 illustrates an example HDAA according to the disclosure.

FIG. 9 shows an example packaged HDAA RF coil array. The RF coil arrayis enclosed in an outer enclosure 910 of suitable material, such as apolyurethane fabric like DARTEX® material. The material 910 iswaterproof, semi-vapor permeable and anti-fungal treated. It is also‘fabric weldable’ or sealed by RF welding to create welded seams and awaterproof finish well suited for medical applications and environments.A RF coil array interfacing cable 920 extends from an interfacingconnector 915 of the RF coil array. The RF coil array interfacing cable920 may be used to connect the HDAA RF coil array to a processor orother component of the MRI system (e.g., controller unit 25). The RFcoil array interfacing cable 920 may include a plurality of baluns 925or continguous/continuous distributed baluns, such as those describedwith respect to FIGS. 10-12.

Thus, FIGS. 7-9 illustrate an example HDAA RF coil array comprised offlexible, deformable RF coils and miniaturized coupling electronics PCBsof the present disclosure. The HDAA may be supported by a flexible andflame-retardant material and then the whole array may be encased on adurable material such as polyurethane. A single (or in some examples,dual) connection may be present to couple the HDAA to the MRI system. Inthis way, a lightweight, flexible RF coil array may be provided that mayimprove patient comfort, improve image quality by conforming to asubject's anatomy, and decrease costs relative to traditional RF coilarrays. The dimensions of the HDAA RF coil array are approximately 66 cmwide by 78 cm long. The diameter of the RF coil loops is approximately14 cm.

The conductor wires and coil loops used in the RF coil or RF coil arraymay be manufactured in any suitable manner to get the desired resonancefrequency for a desired RF coil application. The desired conductor wiregauge, such as 28 or 30 American Wire Gauge (AWG) or any other desiredwired gauge may be paired with a parallel conductor wire of the samegauge and encapsulated with a dielectric material using an extrusionprocess or a three-dimensional (3D) printing or additive manufacturingprocess. This manufacturing process may be environmentally friendly withlow waste and low-cost.

Thus, the RF coil described herein includes a twin lead conductor wireloop encapsulated in a pTFE dielectric that may have no cuts or at leastone cut in at least one of the two parallel conductor wires and aminiaturized coupling electronics PCB coupled to each coil loop (e.g.,very small coupling electronics PCB approximately the size of 2 cm² orsmaller). The PCBs may be protected with a conformal coating or anencapsulation resin. In doing so, traditional components are eliminatedand capacitance is “built in” the integrated capacitor (INCA) coilloops. Interactions between coil elements are reduced or eliminated. Thecoil loops are adaptable to a broad range of MR operating frequencies bychanging the gauge of conductor wire used, spacing between conductorwires, loop diameters, loop shapes, and the number and placement of cutsin the conductor wires.

The RF coil loops described and illustrated herein are transparent inPET/MR applications, aiding dose management and signal-to-noise ratios(SNR). The miniaturized electronic PCB includes decoupling circuitry,impedance inverter circuitry with impedance matching circuitry and aninput balun, and a pre-amplifier. The pre-amplifier sets new standardsin coil array applications for lowest noise, robustness, andtransparency. The pre-amplifier provides active noise cancelling toreduce current noise, boost linearity, and improve tolerance to varyingcoil loading conditions. Additionally, as explained in more detailbelow, a cable harness with baluns for coupling each of the miniaturizedelectronic feedthrough PCBs to the RF coil array connector thatinterfaces with the MRI system may be provided.

The RF coil described herein is exceptionally lightweight, and may weighless than 10 grams per coil element versus 45 grams per coil elementwith General Electric Company's Geometry Embracing Method (GEM) suite offlexible RF coil arrays. For example, a 30-channel RF coil arrayaccording to the disclosure may weigh less than 0.5 kg. The RF coildescribed herein is exceptionally flexible and durable as the coil isextremely simple with very few rigid components and allowing floatingoverlaps. The RF coil described herein is exceptionally low-cost, e.g.,greater than a ten times reduction from current technology. For example,a 30-channel RF coil array could be comprised of components andmaterials of less than $50. The RF coil described herein does notpreclude current packaging or emerging technologies and could beimplemented in RF coil arrays that do not need to be packaged orattached to a former, or implemented in RF coil arrays that are attachedto flexible formers as flexible RF coil arrays or attached to rigidformers as rigid RF coil arrays.

The RF coil array may be supported by fabric and housed in anotherflexible material enclosure, but the RF coil array remains flexible inmultiple dimensions and the RF coils may not be fixedly connected to oneanother. In some examples, the RF coils may be slidably movable relativeto each other, such that varying amounts of overlap among RF coils isacceptable.

As mentioned previously, the RF coil array of the present disclosure maybe coupled to a RF coil array interfacing cable that includescontiguous, distributed baluns or common-mode traps in order to minimizehigh currents or standing waves, independent of positioning. High stressareas of the RF coil array interfacing cable may be served by severalbaluns. Additionally, the thermal load maybe shared through a commonconductor. The inductance of the central path and return path of the RFcoil array interfacing cables are not substantially enhanced by mutualinductance, and therefore are stable with geometry changes. Capacitanceis distributed and not substantially varied by geometry changes.Resonator dimensions are ideally very small, but in practice may belimited by blocking requirements, electric and magnetic fieldintensities, local distortions, thermal and voltage stresses, etc.

FIG. 10 illustrates a block schematic diagram of a continuous commonmode trap assembly 1000 formed in accordance with various embodiments.The common mode trap assembly 1000 may be configured as a transmissioncable 1001 configured for transmission of signals between a processingsystem 1050 and a RF coil array 1060 of an MRI system. Transmissioncable 1001 is a non-limiting example of a RF coil array interfacingcable 212 of FIG. 2, processing system 1050 is a non-limiting example ofcontroller unit 210 of FIG. 2, and RF coil array 1060 is a non-limitingexample of a plurality of RF coils 202 and coupling electronics 203 ofFIG. 2.

In the illustrated embodiment, the transmission cable 1001 (or RF coilarray interfacing cable) includes a central conductor 1010 and pluralcommon mode traps 1012, 1014, 1016. It may be noted that, while thecommon mode traps 1012, 1014, and 1016 are depicted as distinct from thecentral conductor 1010, in some embodiments, the common mode traps 1012,1014, 1016 may be integrally formed with or as a part of the centralconductor 1010.

The central conductor 1010 in the illustrated embodiment has a length1004, and is configured to transmit a signal between the RF coil array1060 and at least one processor of an MRI system (e.g., processingsystem 1050). The central conductor 1010 may include one or more of aribbon conductor, a wire conductor, a planar strip conductor, or acoaxial cable conductor, for example. The length 1004 of the depictedcentral conductor 1010 extends from a first end of the central conductor1010 (which is coupled to the processing system 1050) to a second end ofthe central conductor 1010 (which is coupled to the RF coil array 1060).In some embodiments, the central conductor may pass through a centralopening of the common mode traps 1012, 1014, 1016.

The depicted common mode traps 1012, 1014, 1016 (which may be understoodas cooperating to form a common mode trap unit 1018), as seen in FIG.10, extend along at least a portion of the length 1004 of the centralconductor 1010. In the illustrated embodiment, common mode traps 1012,1014, 1016 do not extend along the entire length 1004. However, in otherembodiments, the common mode traps 1012, 1014, 1016 may extend along theentire length 1004, or substantially along the entire length 1004 (e.g.,along the entire length 1004 except for portions at the end configuredto couple, for example, to a processor or RF coil array). The commonmode traps 1012, 1014, 1016 are disposed contiguously. As seen in FIG.10, each of the common mode traps 1012, 1014, 1016 is disposedcontiguously to at least one other of the common mode traps 1012, 1014,1016. As used herein, contiguous may be understood as includingcomponents or aspects that are immediately next to or in contact witheach other. For example, contiguous components may be abutting oneanother. It may be noted that in practice, small or insubstantial gapsmay be between contiguous components in some embodiments. In someembodiments, an insubstantial gap (or conductor length) may beunderstood as being less than 1/40^(th) of a wavelength of a transmitfrequency in free space. In some embodiments, an insubstantial gap (orconductor length) may be understood as being two centimeters or less.Contiguous common mode traps, for example, have no (or insubstantial)intervening gaps or conductors therebetween that may be susceptible toinduction of current from a magnetic field without mitigation providedby a common mode trap.

For example, as depicted in FIG. 10, the common mode trap 1012 iscontiguous to the common mode trap 1014, the common mode trap 1014 iscontiguous to the common mode trap 1012 and the common mode trap 1016(and is interposed between the common mode trap 1012 and the common modetrap 1016), and the common mode trap 1016 is contiguous to the commonmode trap 1014. Each of the common mode traps 1012, 1014, 1016 areconfigured to provide an impedance to the receive transmitter drivencurrents of an MRI system. The common mode traps 1012, 1014, 1016 invarious embodiments provide high common mode impedances. Each commonmode trap 1012, 1014, 1016, for example, may include a resonant circuitand/or one or more resonant components to provide a desired impedance ator near a desired frequency or within a target frequency range. It maybe noted that the common mode traps 1012, 1014, 1016 and/or common modetrap unit 1018 may also be referred to as chokes or baluns by thoseskilled in the art.

In contrast to systems having separated discrete common mode traps withspaces therebetween, various embodiments (e.g., the common mode trapassembly 1000) have a portion over which common mode traps extendcontinuously and/or contiguously, so that there are no locations alongthe portion for which a common mode trap is not provided. Accordingly,difficulties in selecting or achieving particular placement locations ofcommon mode traps may be reduced or eliminated, as all locations ofinterest may be included within the continuous and/or contiguous commonmode trap. In various embodiments, a continuous trap portion (e.g.,common mode trap unit 1018) may extend along a length or portion thereofof a transmission cable. The continuous mode trap portion may be formedof contiguously-joined individual common mode traps or trap sections(e.g., common mode traps 1012, 1014, 1016). Further, contiguous commonmode traps may be employed in various embodiments to at least one oflower the interaction with coil elements, distribute heat over a largerarea (e.g., to prevent hot spots), or help ensure that blocking islocated at desired or required positions. Further, contiguous commonmode traps may be employed in various embodiments to help distributevoltage over a larger area. Additionally, continuous and/or contiguouscommon mode traps in various embodiments provide flexibility. Forexample, in some embodiments, common mode traps may be formed using acontinuous length of conductor(s) (e.g., outer conductors wrapped abouta central conductor) or otherwise organized as integrally formedcontiguous sections. In various embodiments, the use of contiguousand/or continuous common mode traps (e.g., formed in a cylinder) providefor a range of flexibility over which flexing of the assembly does notsubstantially change the resonant frequency of the structure, or overwhich the assembly remains on frequency as it is flexed.

It may be noted that the individual common mode traps or sections (e.g.,common mode traps 1012, 1014, 1016) in various embodiments may beconstructed or formed generally similarly to each other (e.g., each trapmay be a section of a length of tapered wound coils), but eachindividual trap or section may be configured slightly differently thanother traps or sections. For example, in some embodiments, each commonmode trap 1012, 1014, 1016 is tuned independently. Accordingly, eachcommon mode trap 1012, 1014, 1016 may have a resonant frequency thatdiffers from other common mode traps of the same common mode trapassembly 1000.

Alternatively or additionally, each common mode trap may be tuned tohave a resonant frequency near an operating frequency of the MRI system.As used herein, a common mode trap may be understood as having aresonant frequency near an operating frequency when the resonantfrequency defines or corresponds to a band that includes the operatingfrequency, or when the resonant frequency is close enough to theoperating frequency to provide on-frequency blocking, or to provide ablocking impedance at the operating frequency.

Further additionally or alternatively, each common mode trap may betuned to have a resonant frequency below an operating frequency of theMRI system (or each common mode trap may be tuned to have resonantfrequency above an operating frequency of the MRI system). With eachtrap having a frequency below (or alternatively, with each trap having afrequency above) the operating frequency, the risk of any of the trapscanceling each other out (e.g., due to one trap having a frequency abovethe operating frequency and a different trap having a frequency belowthe operating frequency) may be eliminated or reduced. As anotherexample, each common mode trap may be tuned to a particular band toprovide a broadband common mode trap assembly.

In various embodiments, the common mode traps may have a two-dimensional(2D) or three-dimensional (3D) butterfly configuration to counteractmagnetic field coupling and/or local distortions.

FIG. 11 is a perspective view of a RF coil array interfacing cable 1100including a plurality of continuous and/or contiguous common mode trapsaccording to an embodiment of the disclosure. The RF coil arrayinterfacing cable 1100 includes an outer sleeve or shield 1103, adielectric spacer 1104, an inner sleeve 1105, a first common mode trapconductor 1107, and a second common mode trap conductor 1109.

The first common mode trap conductor 1107 is wrapped in a spiral aboutthe dielectric spacer 1104, or wrapped in a spiral at a taperingdistance from a central conductor (not shown) disposed within the bore1118 of the RF coil array interfacing cable 1100, in a first direction1108. Further, the second common mode trap conductor 1109 is wrapped ina spiral about the dielectric spacer 1104, or wrapped in a spiral at atapering distance from the central conductor disposed within the bore1118, in a second direction 1110 that is opposite to the first direction1108. In the illustrated embodiment, the first direction 1108 isclockwise and the second direction 1110 is counter-clockwise.

The conductors 1107 and 1109 of the RF coil array interfacing cable 1100may comprise electrically-conductive material (e.g., metal) and may beshaped as ribbons, wires, and/or cables, for example. In someembodiments, the counterwound or outer conductors 1107 and 1109 mayserve as a return path for a current passing through the centralconductor. Further, in various embodiments, the counterwound conductors1107 and 1109 may cross each other orthogonally (e.g., a center line orpath defined by the first common mode trap conductor 1107 isperpendicular to a center line or path defined by the second common modetrap conductor 1109 as the common mode trap conductors cross paths) toeliminate, minimize, or reduce coupling between the common mode trapconductors.

It may be further noted that in various embodiments the first commonmode trap conductor 1107 and the second common mode trap conductor 1109are loosely wrapped about the dielectric spacer 1104 to provideflexibility and/or to reduce any binding, coupling, or variation ininductance when the RF coil array interfacing cable 1100 is bent orflexed. It may be noted that the looseness or tightness of thecounterwound outer conductors may vary by application (e.g., based onthe relative sizes of the conductors and dielectric spacer, the amountof bending or flexing that is desired for the common mode trap, or thelike). Generally, the outer or counterwound conductors should be tightenough so that they remain in the same general orientation about thedielectric spacer 1104, but loose enough to allow a sufficient amount ofslack or movement during bending or flexing of the RF coil arrayinterfacing cable 1100 to avoid, minimize, or reduce coupling or bindingof the counterwound outer conductors.

In the illustrated embodiment, the outer shielding 1103 is discontinuousin the middle of the RF coil array interfacing cable 1100 to expose aportion of the dielectric spacer 1104 which in some embodiments isprovided along the entire length of the RF coil array interfacing cable1100. The dielectric spacer 1104, may be comprised, as a non-limitingexample, of TEFLON® or another dielectric material. The dielectricspacer 1104 functions as a capacitor and thus may be tuned or configuredto provide a desired resonance. It should be appreciated that otherconfigurations for providing capacitance to the RF coil arrayinterfacing cable 1100 are possible, and that the illustratedconfigurations are exemplary and non-limiting. For example, discretecapacitors may alternatively be provided to the RF coil arrayinterfacing cable 1100.

Further, the RF coil array interfacing cable 1100 includes a first post1113 and a second post (not shown) to which the first common mode trapconductor 1107 and the second common mode trap conductor 1109 are fixed.To that end, the first post 1113 and the second post are positioned atthe opposite ends of the common mode trap, and are fixed to the outershielding 1103. The first post 1113 and the second post ensure that thefirst and second common mode trap conductors 1107 and 1109 arepositioned close to the outer shielding 1103 at the ends of the RF coilarray interfacing cable 1100, thereby providing a tapered butterflyconfiguration of the counterwound conductors as described furtherherein.

The tapered butterfly configuration includes a first loop formed by thefirst common mode trap conductor 1107 and a second loop formed by thesecond common mode trap conductor 1109, arranged so that an inducedcurrent (a current induced due to a magnetic field) in the first loopand an induced current in the second loop cancel each other out. Forexample, if the field is uniform and the first loop and the second loophave equal areas, the resulting net current will be zero. The taperedcylindrical arrangement of the loops provide improved flexibility andconsistency of resonant frequency during flexing relative totwo-dimensional arrangements conventionally used in common mode traps.

Generally, a tapered butterfly configuration as used herein may be usedto refer to a conductor configuration that is flux cancelling, forexample including at least two similarly sized opposed loops that aresymmetrically disposed about at least one axis and are arranged suchthat currents induced in each loop (or group of loops) by a magneticfield tends to cancel out currents induced in at least one other loop(or group of loops). For example, with reference to FIG. 10, in someembodiments, counterwound conductors (e.g., conductors wound about acentral member and/or axis in opposing spiral directions) may be spaceda distance radially from the central conductor 1010 to form the commonmode traps 1012, 1014, 1016. As depicted in FIG. 11, the radial distancemay be tapered towards the end of the common mode traps to reduce oraltogether eliminate fringe effects. In this way, the common mode traps1012, 1014, 1016 may be continuously or contiguously positioned withoutsubstantial gaps therebetween.

The tapered spiral configuration of the common mode trap conductorsdescribed herein above is particularly advantageous when multiple commonmode trap conductors are contiguously disposed in a common mode trapassembly. As an illustrative example, FIG. 12 is a perspective view of aRF coil array interfacing cable 1250 including a plurality of continuousand/or contiguous common mode traps coupling an RF coil array 1270 to aprocessing system 1260. RF coil array interfacing cable 1250 includes afirst common mode trap 1280 and a second common mode trap 1290positioned adjacent to each other on a central conductor 1252.

The first common mode trap 1280 includes a first common mode trapconductor 1282 and a second common mode trap conductor 1284 counterwoundin a tapered spiral configuration. To that end, the first and secondconductors 1282 and 1284 are fixed to posts 1286 and 1288. It should benoted that the posts 1286 and 1288 are aligned on a same side of thecommon mode trap 1280.

Similarly, the second common mode trap 1290 includes a third common modetrap conductor 1292 and a fourth common mode trap conductor 1294counterwound in a tapered spiral configuration and fixed to posts 1296and 1298. It should be noted that the posts 1296 and 1298 are aligned ona same side of the common mode trap 1290.

As depicted, the common mode traps 1280 and 1290 are separated by adistance, thereby exposing the central conductor 1252 in the gap 1254between the common mode traps. Due to the tapering spiral configurationof the common mode trap conductors of the common mode traps, the gap1254 can be minimized or altogether eliminated in order to increase thedensity of common mode traps in a common mode trap assembly without lossof impedance functions of the common mode traps. That is, the distancecan be made arbitrarily small such that the common mode traps are inface-sharing contact, given the tapered spiral configuration.

It should be appreciated that while the RF coil array interfacing cable1250 includes two common mode traps 1280 and 1290, in practice a RF coilarray interfacing cable may include more than two common mode traps.

Further, the common mode traps 1280 and 1290 of the RF coil arrayinterfacing cable 1250 are aligned such that the posts 1286, 1288, 1296,and 1298 are aligned on a same side of the RF coil array interfacingcable. However, in examples where cross-talk between the common modetraps may be possible, for example if the tapering of the counterwoundconductors is more severe or steep, the common mode traps may be rotatedwith respect to one another to further reduce fringe effects and/orcross-talk between the traps.

Additionally, other common mode trap or balun configurations arepossible. For example, the exterior shielding of each common mode trapmay be trimmed such that the common mode traps can be overlapped orinterleaved, thus increasing the density of the common mode traps.

As used herein, an element or step recited in the singular and proceededwith the word “a” or “an” should be understood as not excluding pluralof said elements or steps, unless such exclusion is explicitly stated.Furthermore, references to “one embodiment” of the present disclosureare not intended to be interpreted as excluding the existence ofadditional embodiments that also incorporate the recited features.Moreover, unless explicitly stated to the contrary, embodiments“comprising,” “including,” or “having” an element or a plurality ofelements having a particular property may include additional suchelements not having that property. The terms “including” and “in which”are used as the plain-language equivalents of the respective terms“comprising” and “wherein.” Moreover, the terms “first,” “second,” and“third,” etc. are used merely as labels, and are not intended to imposenumerical requirements or a particular positional order on theirobjects.

This written description uses examples to disclose the invention,including the best mode, and also to enable a person of ordinary skillin the relevant art to practice the invention, including making andusing any devices or systems and performing any incorporated methods.The patentable scope of the invention is defined by the claims, and mayinclude other examples that occur to those of ordinary skill in the art.Such other examples are intended to be within the scope of the claims ifthey have structural elements that do not differ from the literallanguage of the claims, or if they include equivalent structuralelements with insubstantial differences from the literal languages ofthe claims.

1. A radio frequency (RF) coil array for a magnetic resonance imaging(MRI) system, comprising: a plurality of RF coils, each RF coilcomprising an integrated capacitor coil loop; a plurality of couplingelectronics units each coupled to a respective coil loop; and aplurality of coil-interfacing cables coupling each coupling electronicsunit to an interfacing connector configured to couple to a cable of theMR imaging system.
 2. The RF coil array of claim 1, further comprising asheet of material to which the plurality of RF coils, plurality ofcoupling electronics units, plurality of coil-interfacing cables areattached.
 3. The RF coil array of claim 2, wherein the plurality of RFcoils, plurality of coupling electronics units, plurality ofcoil-interfacing cables, and attached sheet of material are encased in apolyurethane-based enclosure.
 4. The RF coil array of claim 1, whereineach coupling electronics unit comprises a pre-amplifier and animpedance matching network configured to generate a high blockingimpedance.
 5. The RF coil array of claim 4, wherein each couplingelectronics unit further includes a decoupling circuit.
 6. The RF coilarray of claim 4, wherein the pre-amplifier comprises a low inputimpedance pre-amplifier optimized for high source impedance, and whereinthe impedance matching network provides the high source impedance. 7.The RF coil array of claim 1, wherein each integrated capacitor coilloop comprises two parallel wire conductors encapsulated and separatedby a dielectric material.
 8. The RF coil array of claim 7, wherein acapacitance of each coil loop is a function of a spacing between therespective two parallel wire conductors, a position and/or number ofcuts on the two parallel wire conductors, and the dielectric material.9. The RF coil array of claim 7, wherein each coil loop is void of anycapacitive and inductive lumped components along an entire length of thecoil loop between terminating ends thereof.
 10. The RF coil array ofclaim 1, wherein the plurality of RF coils comprises a first RF coil, asecond RF coil, a third RF coil, a fourth RF coil, and a fifth RF coilarranged into a first row with an additional five rows of RF coils for atotal of six rows of five RF coils.
 11. The RF coil array of claim 10,wherein each coil-interfacing cable coupled to each coupling electronicsunit of each RF coil forms a cable assembly that extends a long acentral axis of the RF coil array to the interfacing connector.
 12. TheRF coil array of claim 11, further comprising at least one balunpositioned around the cable assembly.
 13. A radio frequency (RF) coilarray for a magnetic resonance imaging (MRI) system, comprising: aplurality of RF coils arranged into a plurality of rows, each row of RFcoils having an equal number of RF coils, each RF coil comprising anintegrated capacitor coil loop; a plurality of coupling electronicsunits each coupled to a respective coil loop; a plurality ofcoil-interfacing cables coupling each coupling electronics unit to aninterfacing connector configured to couple to a cable of the MR imagingsystem, the coil-interfacing cables from the plurality of RF coilsforming a cable assembly; and at least one balun positioned along thecable assembly.
 14. The RF coil array of claim 13, wherein each couplingelectronics unit is orientated toward a central axis of the RF coilarray, and wherein the cable assembly extends along the central axis.15. The RF coil array of claim 13, wherein the plurality of RF coilscomprises six rows of RF coils, each row comprising five RF coils.